Corrosion-resistant structure for high-temperature water system and corrosion-preventing method thereof

ABSTRACT

The present invention provides a corrosion-resistant structure for a high-temperature water system comprising: a structural material  1;  and a corrosion-resistant film  3  formed from a substance containing at least one of La and Y deposited on a surface in a side that comes in contact with a cooling water  4,  of the structural material  1  which constitutes the high-temperature water system that passes a cooling water  4  of high temperature therein. Due to above construction, there can be provided the corrosion-resistant structure and a corrosion-preventing method capable of operating a plant without conducting a water chemistry control of cooling water by injecting chemicals.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/510,008, filed on May 16, 2012, the text of which is incorporatedby reference, which is a National Stage entry under 35 U. S. C. 371 ofPCT/JP10/070355, filed on Nov. 16, 2010 and claims priority to JapanesePatent Application No. 2009-260933, filed on Nov. 16, 2009.

TECHNICAL FIELD

The present invention relates to a corrosion-resistant structure and acorrosion-preventing method for a high-temperature water system, andparticularly relates to the corrosion-resistant structure and thecorrosion-preventing method for the high-temperature water system, whichcan effectively prevent the corrosion of a structural material thatconstitutes a secondary cooling system of a pressurized-water typenuclear power plant (atomic power generation facility) and caneffectively reduce the elution of a ferrous component and the like fromthe structural material.

BACKGROUND ART

The pressurized-water type nuclear power station (atomic powergeneration facility) is a reactor facility which heats pressurized water(light water with high pressure) that is a primary coolant to 300° C. orhigher with thermal energy generated by a nuclear fission reaction,boils a light water of a secondary coolant with a steam generator toeventually convert the light water into steam of high temperature andhigh pressure, and rotates a turbine generator by using the steam togenerate an electric power. This pressurized-water type reactor is usedfor large-sized plants such as a nuclear power station, and small plantssuch as a nuclear vessel (atomic-powered ship).

In various plants that include the above described pressurized-watertype atomic power generation facility and have a boiler, a steamgenerator, a heat exchanger and/or the like, in which high-temperaturewater circulates, it becomes a big problem that ions elute from themetal of the structural material or the structural material itselfcorrodes. The elution of the metal ions is a representative phenomenonoccurring in the high-temperature water, and the elution causes thecorrosion of structural members of pipes and equipments, including thestructural material, and eventually gives various influences such as anoperational problem and the increase of maintenance frequency, on theplant.

In addition, the eluted metal ions from the structural material and thelike adhere to and deposit on a surface of the pipes in the system, or ahigh-temperature site of the steam generator and the like, as an oxide,and there is a possibility that impurities form a highly concentratedstate, in a narrow portion such as a crevice portion between a heattransfer tubing and a tube-support-plate in a heat exchanger. Theimpurities also may form an ion-enriched water having strong acidity orstrong alkalinity according to the ion balance, and further causeremarkable corrosion.

A phenomenon of corrosion cracking in the structural material is alsoconfirmed which is caused by such a phenomenon and a rise of anelectrochemical potential due to the oxide which adheres to the surface.Heat transfer also decreases due to the adhering oxide, and accordinglyit is needed to remove the oxide on the structural material by chemicalcleaning or the like periodically with a high frequency.

On the other hand, there has been a high possibility in recent yearsthat the thickness of a carbon steel pipe decreases due to awall-thinning phenomenon of the pipe and such an accident that the pipeis ruptured also occurs. Thus, the elution, the corrosion phenomenon andthe like of the metal are accumulated with time during a plant operationin a long period of time, and potentially show a possibility of suddenlyerupting into a disaster at some point when the accumulated amount hasreached to a durable limit.

Furthermore, the above described corrosion rate is accelerated dependingon a shape of a structural site, and a phenomenon which is difficult tobe predicted may occur. For instance, in a piping system in which manyequipments such as an orifice and a valve are used, erosion or corrosionis caused by the flow of a fluid of high temperature such as a coolingwater which passes through the inner space at a high speed. In order toavoid such a problem, various corrosion mitigation methods including awater chemistry control have been conventionally implemented in variousplant systems.

For instance, in the secondary cooling system of a thermal power stationand a pressurized-water type nuclear power station, such measures aretaken as to control a pH in a cooling water by injecting ammonia orhydrazine, thereby decrease the elution of iron from the inside of thesystem and prevent the inflow of the iron component to the steamgenerator (Patent Literature 1).

Furthermore, in order to eliminate the enrichment of alkaline componentsin the crevice portion, various water chemistry controls have beenimplemented in an actual plant, such as the control of an Na/Cl ratio,the control of chloride ion concentration for decreasing an influence ofa chlorine ion on corrosion, and the control of dissolved oxygenconcentration (Patent Literature 2). In recent years, a water chemistrycontrol method is also adopted which uses improved chemicals such asethanolamine and morpholine.

As described above, various technologies for controlling the waterchemistry have been proposed as an improved proposal, in addition to themeasures which have been already implemented in the actual plant, suchas reductions of the corrosion of pipes, the adhesion and deposition ofan oxide and the like, and the enrichment of eluted components in thecrevice portion. As for the improvement of the chemicals to be injected,for instance, there is a method of using an organic acid such as tannicacid and ascorbic acid as an oxygen scavenger (Patent Literature 3).

In addition, as for the water chemistry control method, there areproposed an operation method of controlling a molar ratio of allcations/SO₄ (Patent Literature 2), a method of introducing at least oneof a calcium compound and a magnesium compound into feed water to asteam generator for a reactor so that the ion concentration becomes 0.4to 0.8 ppb (Patent Literature 2), and the like.

Thus, the measures of suppressing corrosion and elution by waterchemistry control with the use of the chemicals are widely implementedunder present circumstances as a measure of preventing the corrosion andelution of a plant structural material. However, such a technology isdesired which can operate the plant without controlling a waterchemistry of the cooling water by injecting the chemicals, from theviewpoints of the complexity of operation management, an operation costand the safety.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent No. 2848672

Patent Literature 2: Japanese Patent No. 3492144

Patent Literature 3: Japanese Patent Laid-Open No. 2004-12162

SUMMARY OF INVENTION Problems to Be Solved by the Invention

A present secondary cooling system of a pressurized-water type atomicpower generation facility is operated in a state of having a chemicalagent such as hydrazine and ammonium injected therein so as to suppressits corrosion. A new technology is necessary in order to enable theplant to be operated without the injection of the chemicals.

Then, an object of the present invention is to provide acorrosion-resistant structure and a corrosion-preventing method for ahigh-temperature water system, which can easily operate the plant whileobtaining an effective corrosion-preventing effect, not by controllingthe water chemistry of a cooling water by injecting the chemicals intothe structure, but by providing a technology of modifying a surface of astructural material.

Means for Solving the Problems

In order to achieve the above described object, a corrosion-resistantstructure for a high-temperature water system according to oneembodiment of the present invention has a corrosion-resistant filmformed from a substance containing at least one of La and Y deposited ona surface in a side that comes in contact with a cooling water, of astructural material which constitutes the high-temperature water systemthat passes a cooling water of high temperature therein.

The corrosion-resistant film which is formed from the substancecontaining at least one of La and Y and has deposited on the surface caneffectively prevent the corrosion of the structural material, and cangreatly reduce the elution of a metal component such as iron from acooling water contact surface of the structural material.

In the corrosion-resistant structure for the high-temperature watersystem, the temperature of the cooling water of high temperature ispreferably 20° C. or higher and 350° C. or lower. The above describedcorrosion-preventing effect of the corrosion-resistant film which hasdeposited on the surface of the structural material shows ananticorrosive effect in a wide temperature range from the abovedescribed ordinary temperature to an operation temperature of thesecondary cooling system of the pressurized-water type atomic powergeneration facility.

Furthermore, in the above corrosion-resistant structure of thehigh-temperature water system, the substance containing La is preferablyat least one La compound selected from La₂O₃, La(OH)₃, La₂(CO₃)₃,La(CH₃COO)₃ and La₂(C₂O4)₃. Any one of these La compounds shows anexcellent anticorrosive effect when being contained in thecorrosion-resistant film.

In the corrosion-resistant structure for the high-temperature watersystem, the substance containing Y is preferably at least one Y compoundselected from Y(OH)₃, Y₂(CO₃)₃, Y(CH₃COO)₃ and Y₂(C₂O₄)₃. Any one ofthese Y compounds shows an excellent anticorrosive effect when beingcontained in the corrosion-resistant film, though the effects aredifferent to some extent according to the type.

In the corrosion-resistant structure for the high-temperature watersystem, the structural material (structural member) is preferably atleast one structural material selected from a carbon steel, a copperalloy and an Ni-based alloy. Any one of the carbon steel, the copperalloy and the Ni-based alloy can effectively prevent the elution of itsmetal component even though the above described structural material isany one of them.

In the corrosion-resistant structure for the high-temperature watersystem, the deposition amount of La is preferably 1 μg/cm² or more and200 μg/cm² or less. When the deposition amount of La is in the abovedescribed range, a high corrosion-preventing effect can be obtained. Onthe other hand, even when the deposition amount of La exceeds the upperlimit of the above described range, the corrosion-preventing effectresults in being saturated.

Furthermore, in the above corrosion-resistant structure for thehigh-temperature water system, the deposition amount of Y is preferably1 μg/cm² or more and 200 μg/cm² or less. When the deposition amount of Yis in the above described range, a high corrosion-preventing effect isobtained. On the other hand, even when the deposition amount of Yexceeds the upper limit of the above described range, thecorrosion-preventing effect results in being saturated, similarly to theLa compound.

In addition, a corrosion-preventing method for a high-temperature watersystem according to the present invention for preventing a corrosion ofa structural material constituting the high-temperature water systemthrough which a cooling water of high temperature passes includes stepsof: preparing a corrosion inhibitor containing at least one of La and Y;and depositing a prepared corrosion inhibitor on a surface in a side ofthe structural material, which comes in contact with the cooling water,and forming a corrosion-resistant film thereon.

In the above description, it is preferable to previously subject asurface in a side on which the structural material comes in contact withthe cooling water, to any one treatment among machining treatment,immersion treatment in high-temperature water and chemical cleaningtreatment, before depositing the corrosion-resistant film. In otherwords, when a cooling water contact surface of the structural materialis previously subjected to the machining treatment such as grinding by aliner or the like, thereby an oxide film and a foreign substance of thesurface portion are removed and a newly-formed surface is made toappear, the newly-formed surface can enhance an adhesion strength of thecorrosion-resistant film.

In addition, it is preferable that the structural material is subjectedto the treatment of immersion into a high-temperature water of 200° C.to 350° C., thereby an oxide film of the structural material is formedon the surface of the structural material (substrate, base member) andthe corrosion resistant film is formed on the surface of this oxidefilm. This oxide film further enhances a function of thecorrosion-resistant film containing La and Y, and can further enhancethe corrosion-preventing effect.

Furthermore, when the structural material is previously subjected to achemical cleaning treatment of cleaning the cooling water contactsurface of the structural material with an acid or the like, thereby toremove the oxide and the foreign substance and to make a newly-formedsurface appear, the newly-formed surface can enhance an adhesionstrength of the corrosion-resistant film, similarly to the abovedescribed case of the structural material which has been subjected tothe machining treatment.

In addition, in the above described corrosion-preventing method for thehigh-temperature water system, the above described method of depositingthe corrosion inhibitor on the surface of the structural material ispreferably any one of a spray method, a CVD method, a thermal spraymethod and an immersion method in which the structural material isimmersed into a high-temperature water containing the corrosioninhibitor.

The above described spray method is a method of spraying the corrosioninhibitor onto the surface of the structural material with a highpressure gas such as nitrogen gas; the CVD method is a method ofchemically vaporizing the corrosion inhibitor, and vapor-depositing thecorrosion inhibitor on the surface of the structural material; thethermal spray method is a method of spraying a melted corrosioninhibitor onto the surface of the structural material so as to cover thesurface with the melted corrosion inhibitor; and the immersion method isa method of immersing the structural material into the high-temperaturewater containing the corrosion inhibitor and depositing the corrosioninhibitor on the surface of the structural material. Any method can bemore promptly and easily applied to the structural material, incomparison with a conventional operation of controlling a waterchemistry of a cooling material.

Advantageous Effects of the Invention

According to the corrosion-resistant structure and thecorrosion-preventing method for the high-temperature water system of thepresent invention, a corrosion-resistant film formed from a substancecontaining at least one of La and Y is deposited on a surface of astructural material, accordingly the structural material can beeffectively prevented from causing corrosion, and an elution of a metalcomponent such as iron from the cooling water contact face of thestructural material can be greatly reduced. In addition, the abovedescribed corrosion-resistant film shows an excellentcorrosion-preventing effect even when the deposition amount is small,and on the other hand, maintains the corrosion-preventing effect for along period of time because of having high adhesion strength between thecorrosion-resistant film and the structural material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are cross-sectional views illustrating examples of acorrosion-suppressing structure for conducting a corrosion-resistantstructure and a corrosion-preventing method for a high-temperature watersystem according to the present invention; FIG. 1A is a view of anexample in which a corrosion-resistant film containing La is formed on asurface of a structural material (substrate, base member) having anoxide film formed thereon; and FIG. 1B is a cross-sectional viewillustrating an example in which a corrosion-resistant preventing filmcontaining La is directly formed on a surface of a structural materialfrom which an oxide film has been removed.

FIG. 2 is a graph illustrating a relationship between acorrosion-suppressing effect and the corrosion-resistant structure, inthe corrosion-resistant structures illustrated in FIGS. 1A and 1B.

FIG. 3 is a graph illustrating a corrosion-suppressing effect of acorrosion-resistant structure which has a corrosion-resistant filmformed from Y(OH)₃ formed thereon.

FIG. 4 is a graph illustrating an influence of a temperature change of acorrosion-resistant structure having the corrosion-resistant film formedfrom Y(OH)₃ formed thereon.

FIG. 5 is a graph illustrating a relationship between deposition amountsof corrosion-resistant films and corrosion amounts (corrosion rate) ofstructural members.

FIG. 6 is a graph illustrating a relationship between a method forforming the corrosion-resistant film and a corrosion amount of astructural member.

FIG. 7 is a graph illustrating a relationship between the type ofchemical compounds (corrosion inhibitor) contained incorrosion-resistant films and the corrosion-suppressing effect.

MODE FOR CARRYING OUT THE INVENTION

Examples of the corrosion-resistant structure and thecorrosion-preventing method for the high-temperature water systemaccording to the present invention will be more specifically describedhereinbelow with reference to the attached drawings.

EXAMPLE 1

Firstly, an example of the present invention in which acorrosion-resistant film containing a La compound as a corrosioninhibitor is formed on a structural material will be concretelydescribed below with reference to the attached FIGS. 1A and 1B and FIG.2.

A corrosion-resistant structure for a high-temperature water systemaccording to the present example 1 includes two types of structures, asare illustrated in FIGS. 1A and 1B and FIG. 2. Specifically, FIG. 1A isa view of an example in which a corrosion-resistant film 3 formed fromLa₂O₃ has been formed on the surface of a carbon steel that is used as astructural material (substrate, base member) 1 and has a uniform oxidefilm 2 formed thereon; and FIG. 1B is a view illustrating an example(test piece) in which the corrosion-resistant film 3 formed from La₂O₃has been directly formed on a surface of the structural material 1 fromwhich an ununiform oxide film has been previously removed.

For information, the oxide film 2 in FIG. 1 A was formed by oxidizing asurface portion of the carbon steel which was used as the structuralmaterial 1, in the atmosphere of 150° C. In addition, a carbon steel 1that was used as the structural material in FIG. 1B had a newly-formedsurface exposed thereon which had a smooth and uniform surfaceroughness, by acid-pickling the surface.

Next, a test piece was prepared as a Comparative Example (reference)which was formed only from a carbon steel and did not have an oxide filmand a corrosion-resistant film formed thereon, in addition to the twotypes of the examples in which the corrosion-resistant film was preparedby depositing La₂O₃ on the carbon steel as was described above. Thesurface portions of these three types of the test pieces were subjectedto a corrosion test under conditions of being immersed in the hot waterwhich contained less than 5 ppb of dissolved oxygen and had a pH of 9.8at a temperature of 185° C. under a pressure of 4 MPa, for 500 hours.Corrosion amounts (corrosion rates) were calculated from weight changesbefore and after the corrosion test of each test piece. The measurementcalculation results are shown in FIG. 2.

As is clear from the result illustrated in FIG. 2, it was proved thatthe corrosion rates were remarkably suppressed in the two types of thetest pieces in the example in which the corrosion-resistant film 3formed from La₂O₃ was deposited, in comparison with the test pieceformed only from the carbon steel. In addition, it was also confirmedthat the corrosion-suppressing effect became more remarkable when theoxide film 2 existed. Thus, it was proved that the corrosion-suppressingfunction for the carbon steel could be effectively shown by La₂O₃ whichwas deposited on the surface of the structural material.

It is expected according to the above described experimental resultsthat an effect of suppressing general corrosion due to a cooling waterand an effect of suppressing a wall thinning phenomenon due toflow-accelerated corrosion can be exhibited by an La-containing compoundwhich has been deposited on a surface of a carbon steel materialconstituting a secondary cooling system of a pressurized-water typeatomic power generation facility.

For information, it is confirmed by an experiment that the abovedescribed corrosion-preventing effect is not limited to the case inwhich La₂O₃ was used as the corrosion inhibitor but the similar effectcan be shown also in the case in which La(OH)₃, La₂(CO₃)₃, La(CH₃COO)₃or La₂(C₂O₄)₃ was used as the corrosion inhibitor to be deposited on thesurface.

EXAMPLE 2

Next, an example of the present invention, in which acorrosion-resistant film containing a Y compound as a corrosioninhibitor has been formed on a structural material, will be describedbelow with reference to the attached FIG. 3.

A corrosion resistant structure for a high-temperature water systemaccording to the present example has a structure as is illustrated in aschematic view FIG. 1B. Specifically, a surface of a test piece of thepresent example is a newly-formed surface which is exposed by removingthe oxide film with chemicals. Y(OH)₃ was used as a corrosion inhibitor.

Then, a corrosion-resistant film 3 was formed with the use of a spraycoating method of spraying a chemical agent containing Y(OH)₃ onto thecooling water contact surface of a carbon steel together with nitrogengas and depositing the chemical agent. As a result of having examined astate of the formed corrosion-resistant film 3 through SEM observation,it was confirmed that a spot-shaped lump of Y(OH)₃ of a micrometricorder was formed on a surface portion of the carbon steel. It was provedfrom this observation result that the deposition uniformity of thecorrosion-resistant film 3 was low and the deposition amount of Y(OH)₃was 90 μg/cm², but that the film thickness considerably dispersed orscattered depending on the site of the carbon steel.

Next, a test piece was prepared as a Comparative Example (reference)which was formed only from a carbon steel and did not have an oxide filmand a corrosion-resistant film formed thereon, in addition to theexample in which the corrosion-resistant film was prepared by depositingY(OH)₃ on the carbon steel as was described above. The surface portionsof these two types of the test pieces were subjected to a corrosion testunder conditions of being immersed in the hot water which contained lessthan 5 ppb of dissolved oxygen and had a pH of 9.8 at a temperature of185° C. under a pressure of 4 MPa, for 500 hours, in a similar way tothat in Example 1. Corrosion amounts (corrosion rates) were calculatedfrom weight changes before and after the corrosion test of each testpiece. The measurement calculation results are shown in FIG. 3.

As is clear from the result illustrated in FIG. 3, it was proved thatthe corrosion rate was suppressed to approximately one-tenth in the testpiece in Example 2 in which the corrosion-resistant film formed fromY(OH)₃ was deposited, and that an excellent corrosion-preventing effectcould be shown, in comparison with the test piece formed only from thecarbon steel. Thus, it was proved that the corrosion-suppressingfunction for the carbon steel could be effectively shown by Y(OH)₃ whichwas deposited on the surface of the structural material.

It is expected on the basis of the above described experimental resultthat an effect of suppressing general corrosion of the structuralmaterial and an effect of suppressing a wall thinning phenomenon due toflow-accelerated corrosion are shown when Y(OH)₃ has been deposited on asurface of a structural material constituting a secondary cooling systemof a pressurized-water type atomic power generation facility.

In addition, it is confirmed by an experiment that the above describedcorrosion-preventing effect is not limited to the case in which Y(OH)₃was used as a corrosion inhibitor, but that the similar effect can beshown also in the case in which Y₂(CO₃)₃, Y(CH₃COO)₃ or Y₂(C₂O₄)₃ wasused as the corrosion inhibitor to be deposited on the surface of thestructural material.

EXAMPLE 3

Next, an influence which a difference of an operation temperature(temperature of cooling water) gives on a corrosion-resistant structurewill be described below with reference to the following Example 3 andFIG. 4.

A corrosion-resistant structure for a high-temperature water systemaccording to the present Example 3 has a structure as is illustrated ina schematic view FIG. 1B. Specifically, a test piece used for the testpiece of the present example was in such a state that the surface of acarbon steel before a corrosion-resistant film was deposited thereon hadbeen polished and degreased by a sandpaper with #600, and that an oxidefilm and a foreign substance had been removed therefrom.

Then, the test piece according to Example 3 was prepared by depositingY(OH)₃ onto the surface (newly-formed surface) of this carbon steel witha spray method. A deposition amount of Y(OH)₃ in this test piece was setat 50 μg/cm² by adjustment of a spraying period of time. As a result ofhaving examined a state of the formed corrosion-resistant film 3 throughSEM observation, the uniformity was low similarly to that in Example 2.

Next, a test piece was prepared as a Comparative Example which wasformed only from a carbon steel and did not have an oxide film and acorrosion-resistant film formed thereon, in addition to the example inwhich the corrosion-resistant film was prepared by depositing Y(OH)₃ onthe carbon steel as was described above.

Then, the surface portions of these two types of the test pieces weresubjected to a corrosion test under conditions of being immersed in thehot water which contained 5 ppb or less of dissolved oxygen and had a pHof 9.8 at a temperature in two levels of 150° C. and 280° C. under apressure of 4 MPa and 8 MPa, for 500 hours, in a similar way to that inExample 1. Corrosion amounts (corrosion rates) were calculated fromweight changes before and after the corrosion test of each test piece.The measurement calculation result is shown in FIG. 4.

As is clear from the result illustrated in FIG. 4, the corrosion amountof the test piece formed only from the carbon steel also decreases underthe condition that the temperature is as high as 280° C. This isconsidered to be because the formed oxide film has high stabilitybecause the temperature is high.

On the other hand, it is understood that the corrosion rate becomeslarge when the temperature is 150° C. because the solubility of theoxide film to be formed under the condition of the present test is high,and that the corrosion-suppressing function works due to the depositionof Y(OH)₃. Therefore, the corrosion-resistant structure can be appliedin such an environment that a cooling water is 20° C. or higher and 350°C. or lower which is an operation temperature of a secondary coolingsystem of a pressurized-water type atomic power generation facility, inview of the fact that Y(OH)₃ is resistant to high temperature.

In addition, as is clear from FIG. 4, the corrosion-resistant structureaccording to the present example is particularly effective in a range ofan operation temperature of 150° C. or higher after a deaerator, in asecondary cooling system of a pressurized-water type atomic powergeneration facility, and it is expected that an effect of suppressing angeneral corrosion of a structural material and a function of suppressinga wall thinning phenomenon due to flow-accelerated corrosion areeffectively shown when a chemical agent containing Y is injected intothe system and is deposited on a surface of a structural material.

EXAMPLE 4

Next, an influence which a difference of a deposition amount of acorrosion inhibitor to be deposited on a surface of a structuralmaterial gives on a corrosion amount will be described below withreference to the following Example 4 and FIG. 5.

A corrosion-resistant structure for a high-temperature water systemaccording to the present Example 4 has a structure as is illustrated ina schematic view FIG. 1B. Specifically, a test piece used for the testpiece of the present Example 4 was in such a state that the surface of acarbon steel before a corrosion-resistant film was deposited thereon hadbeen polished and degreased by a sandpaper with #600, and an oxide filmand a foreign substance had been removed therefrom.

Then, a large number of two types of test pieces according to Example 4were prepared by depositing La₂O₃ or Y(OH)₃ onto the surface(newly-formed surface) of this carbon steel with a spray method. Forinformation, a deposition amount of La₂O₃ or Y(OH)₃ was varied andadjusted in a range of 0 to 300 μg/cm² by adjustment of a sprayingperiod of time.

Next, a test piece was prepared as a Comparative Example which wasformed only from a carbon steel and did not have an oxide film and acorrosion-resistant film formed thereon, in addition to the example inwhich the corrosion-resistant film was prepared by depositing La₂O₃ orY(OH)₃ on the surface of the carbon steel as was described above.

Then, the surface portions of these test pieces were subjected to acorrosion test under conditions of being immersed in the hot water whichcontained 5 ppb or less of dissolved oxygen and had a pH of 9.8 at atemperature of 185° C. under a pressure of 4 MPa, for 500 hours, in asimilar way to that in Example 1. Corrosion amounts (corrosion rates)were calculated from weight changes before and after the corrosion testof each test piece. The measurement calculation result is shown in FIG.5.

As is clear from the result illustrated in FIG. 5, it was confirmed thatthe corrosion amount tended to decrease and the corrosion-suppressingeffect tended to increase, as the deposition amount of thecorrosion-resistant film increased. It was also confirmed that thecorrosion-suppressing effect was saturated and the corrosion ratesreached approximately a same level, in a range of a deposition amount of20 μg/cm² or more. Accordingly, the deposition amount of thecorrosion-resistant film is necessary and sufficient to be in the rangeof 20 to 120 μg/cm².

Here, a deposition amount of the corrosion inhibitor remaining on asurface of the test piece of which the deposition amount had been set toapproximately 50 μg/cm² before the corrosion test was examined after thecorrosion test, and as a result, it was confirmed that the depositionamount was 1 μg/cm² or less.

As a result, it was confirmed that the corrosion-preventing effectcontinued as long as a fixed deposition amount of an La-containing orY-containing chemical agent was attained in an initial stage of theapplication, even though the deposition amount was not always keptconstant or the deposition amount decreased due to an exfoliation of thedeposited chemical agent during an operation period.

It is technically difficult to uniformly deposit the present corrosioninhibitor on the surface of the structural material of the secondarycooling system of the pressurized-water type atomic power generationfacility so that the deposition amount becomes uniform, and it isanticipated that the deposition amount of the corrosion inhibitorgreatly varies according to an influence of a flow of a cooling water,and depending on a temperature of the cooling water and a structure ofthe high-temperature water system.

However, such a technological knowledge is an important premise for thetechnology that an initial corrosion-preventing effect develops evenwhen the deposition amount of the corrosion inhibitor has greatly varieddepending on the site of the structural body as has been describedabove, and is extremely useful when the technology is applied to anactual apparatus.

EXAMPLE 5

Next, an influence which a difference between methods of depositing acorrosion inhibitor on a surface of a structural material gives will bedescribed below with reference to the following Example 5 and FIG. 6.

A corrosion-resistant structure for a high-temperature water systemaccording to the present Example 5 has a structure as is illustrated ina schematic view FIG. 1B. Specifically, a test piece used for the testpiece of the present Example 5 was in such a state that the surface of acarbon steel before a corrosion-resistant film was deposited thereon hadbeen polished and degreased by a sandpaper with #600, and an oxide filmand a foreign substance had been removed therefrom.

Then, two types of test pieces according to Example 5 were prepared bydepositing La₂O₃ onto the surface (newly-formed surface) of this carbonsteel with a spray method or a chemical deposition method of injecting achemical substance into a high-temperature water and depositing thechemical substance. In the above description, the deposition amount ofLa₂O₃ was adjusted to 50 μg/cm² by adjustment of a spraying period oftime or an amount of the chemical agent to be injected into thehigh-temperature water.

Here, the above described chemical deposition method is a method ofmaking a substance to be deposited exist in a fluid, and depositing thesubstance onto a surface of a structural material by a flow of thefluid.

Next, the surface portions of the two types of the test pieces whichwere prepared by depositing La₂O₃ on the surface of the carbon steelwith different methods as was described above were subjected to acorrosion test under conditions of being immersed in the hot water thatcontained 5 ppb or less of dissolved oxygen and had a pH of 9.8 at atemperature of 185° C. under a pressure of 4 MPa, for 500 hours, in asimilar way to that in Example 1. Then, corrosion amounts (corrosionrates) were calculated from weight changes before and after thecorrosion test of each test piece. The measurement calculation result isshown in FIG. 6.

As is clear from the result illustrated in FIG. 6, thecorrosion-resistant film which had been deposited and formed with thechemical deposition method was different from and could be moreuniformly deposited than the corrosion-resistant film which had beenformed with the spray method, and it was confirmed that thecorrosion-resistant film which had been formed with the chemicaldeposition method had a greater corrosion-rate-suppressing function.

It is expected that the deposition of the corrosion-resistant filmhaving high uniformity can be achieved by injecting an La-containingsubstance into a high-temperature cooling water during an operation ofthe secondary cooling system of the pressurized-water type atomic powergeneration facility and by depositing the substance onto the surface ofthe structural material, and that thereby an effect of suppressinggeneral corrosion and an effect of suppressing a wall-thinningphenomenon due to flow-accelerated corrosion are shown. A similar effectcan be shown also when a Y-containing substance has been injected intothe high-temperature cooling water.

EXAMPLE 6

Next, an effect appearing when La(OH)₃ or Y₂(CO₃)₃ as other corrosioninhibitors has been deposited on a surface of a structural material willbe described below with reference to the following Example 6 and FIG. 7.

A corrosion-resistant structure for a high-temperature water systemaccording to the present Example 6 has a structure as is illustrated ina schematic view FIG. 1B. Specifically, a test piece used for the testpiece of the present Example 6 was in such a state that the surface of acarbon steel before a corrosion-resistant film was deposited thereon hadbeen polished and degreased by a sandpaper with #600, and an oxide filmand a foreign substance had been removed therefrom.

Then, two types of test pieces according to Example 6 were prepared bydepositing La(OH)₃ or Y₂(CO₃)₃ onto the surface (newly-formed surface)of this carbon steel with the use of a spray method. For information, adeposition amount of La(OH)₃ or Y₂(CO₃)₃ was adjusted to 50 μg/cm² byadjustment of a spraying period of time.

Next, the surface portions of the two types of the test pieces whichwere prepared by depositing La(OH)₃ or Y₂(CO₃)₃ on the surface of thecarbon steel as was described above were subjected to a corrosion testunder conditions of being immersed in the hot water that contained 5 ppbor less of dissolved oxygen and had a pH of 9.8 at a temperature of 185°C. under a pressure of 4 MPa, for 500 hours, in a similar way to that inExample 1. Then, corrosion amounts (corrosion rates) were calculatedfrom weight changes before and after the corrosion test of each testpiece. The measurement calculation result is shown in FIG. 7.

As is clear from the results illustrated in FIG. 7, when the corrosionamounts of the two types of the test pieces which were prepared bydepositing La(OH)₃ or Y₂(CO₃)₃ on the surface of the carbon steel werecompared to each other, the corrosion amounts were not greatly differentfrom each other, but it was confirmed that when the two types of thetest pieces were compared to the test piece formed only from the carbonsteel illustrated in Examples 1 and 2, the corrosion rates wereremarkably suppressed.

It was experimentally proved that a great corrosion-preventing effectwas obtained by depositing and forming a hydroxide of La or a carbonateof Y on the surface of the structural material as in the above describedExample 6. Accordingly, it is expected that an effect of suppressinggeneral corrosion of the structural material and an effect ofsuppressing a wall thinning phenomenon due to flow-accelerated corrosionare shown also when the hydroxide and the carbonate are deposited on thesurface of the structural material in the secondary cooling system ofthe pressurized-water type atomic power generation facility.

INDUSTRIAL APPLICABILITY

According to the corrosion-resistant structure and thecorrosion-preventing method for the high-temperature water system of theembodiments of the present invention, a corrosion-resistant film formedfrom a substance containing at least one of La and Y is deposited on thesurface of the structural material, accordingly the structural materialcan be effectively prevented from causing corrosion, and an elution of ametal component such as iron from the cooling water contact surface ofthe structural material can be greatly reduced. In addition, the abovedescribed corrosion-resistant film shows an excellentcorrosion-preventing effect even when the deposition amount is small,and on the other hand, can maintain the corrosion-preventing effect fora long period of time because of having high adhesion strength betweenthe corrosion-resistant film and the structural material.

DESCRIPTION OF SYMBOLS

-   1 Structural material (carbon steel)-   2 Oxide film (Oxide layer)-   3 Corrosion-preventing film (La₂O₃ film, Y(OH)₃ film, La(OH)₃ film    or Y₂(CO₃)₃ film)-   4 Cooling water (Coolant)

1-9. (canceled)
 10. A corrosion-resistant structure for ahigh-temperature water system, comprising: a structural material; and acorrosion-resistant film formed from a substance comprising Y depositedon a surface in a side that comes in contact with a cooling water havinga temperature of 20° C. or higher and 350° C. or lower, of thestructural material which constitutes the high-temperature water systemthat passes a cooling water of high temperature therein.
 11. Thecorrosion-resistant structure for the high-temperature water systemaccording to claim 10, wherein the substance containing Y is at leastone chemical compound selected from the group consisting of Y(OH)₃,Y₂(CO₃)₃, Y(CH₃COO)₃ and Y₂(C₂O₄)₃.
 12. The corrosion-resistantstructure for the high-temperature water system according to claim 10,wherein the structural material is at least one structural materialselected from the group consisting of a carbon steel, a copper alloy anda Ni-based alloy.
 13. The corrosion-resistant structure for thehigh-temperature water system according to claim 10, wherein adeposition amount of the substance comprising Y is 1 μg/cm² or more and200 μg/cm² or less.
 14. The corrosion-resistant structure for thehigh-temperature water system according to claim 10, wherein an oxidefilm of the structural material is formed on a surface of the structuralmaterial, and the corrosion-resistant film is formed on a surface of theoxide film.